Electrostatic developer

ABSTRACT

An electrostatic developer has means controlled by the displacement of electrographic paper to vary the quantity of developing liquid contacting the paper such that the paper remains in contact with the liquid for a substantially constant time. Such variation can be affected by varying the effective size of a chamber containing developing liquid, by varying the level of liquid in the chamber or by masking nozzles arranged to direct developing liquid towards the paper.

Umted States Patent 1 1 1 3,902,451

Allard Sept. 2, 1975 [54] ELECTROSTATIC DEVELOPER 3,188,649 6/1965 Preisinger et a1. 1l8/DlG. 23 3,367,791 2/1968 Lein 117/37 [751 lnvemo Emmame' Allard Pans France 3,402,695 9/1968 Baker et a1. 118/7 Assigneez Societe dite Allco, n i ini 3,711,312 1/1973 Yosh1da et a1 118/629 France Prima ExaminerMervin Stein F 1 A 28, 1973 W [22] ug Assistant Examiner-Douglas' Salser [21] Appl' N011 392,310 Attorney, Agent, or Firm-Eyre, Mann & Lucas [30] Foreign Appllcatlon Prlorlty Data ABSTRACT Sept. 5, 1972 France 72.32070 Mar. 2, 1973 France 73.08217 An electrostatic developer has means controlled by the displacement of electrographic paper to vary the [52] US. Cl. 118/8; 118/301; 118/504; quantity of developing liquid contacting the paper 1 18/629; 1 18/637; 1 l8/D1G. 23 such that the paper remains in contact with the liquid [51] Int- Cl.- --B05C 6036 B053 N28 for a substantially constant time. Such variation can [58] Field of Search 1 18/301, 504, 627, 629, be affected by varying the effective size of a chamber 118/7, 637, DIG. 23, 8 containing developing liquid, by varying the level of liquid in the chamber or by masking nozzles arranged [56] References Cited to direct developing liquid towards the paper.

UNITED STATES PATENTS 30 Cl 13 D F 3,094,914 6/1963 Smith 1l8/DIG. 23 rawmg gums I v 1 2 1 O 1 9 2 A 7 4' 6 1 k t I 40 a, \11: "ti-1E z-. Z Z/Z' /%4 a 11 3 3 PATENTEU SEP 2191s sum u 0F 5 PATENTEU 2 75 suausp g,

ELECTROSTATIC DEVELOPER BACKGROUND OF THE INVENTION 1. Field of the Invention.

The present invention relates to electrostatic developers.

2. Description of the Prior Art.

In an electrostatic developer, developing liquid (which is termed toner" liquid) is used to develop images of charges deposited on electrographic paper, that is to say a semiconductor base paper coated with a dielectric layer.

Such developers are used, for example in electrostatic printing machines. In an electrostatic developing process, it is necessary that the paper moves at a constant speed at least during its passage through the developing liquid. There is normally an optimum period of contact between the developing liquid and the paper. The contrast of the developed image is reduced for periods other than the optimum period. If the paper remains in contact beyond this period, an increasingly dark grey background is formed. Normally, the optimum period of contact is about 0.5 seconds. Continuous displacement of the paper is disadvantageous when the printing machine is connected on line to a computer. A computer frequently performs a variety of tasks and is not available at all times to feed the printing machine whereby the printing machine stops and awaits the next burst of data, whose arrival is unpre dictable. This operation is referred to as asynchronous operation. If the printing machine is operated in this manner, assuming that the computer feeds same with complete lines or with groups of complete lines, a different contrast is obtained for different groups of lines and, more seriously :1 dark stripe is obtained at each print-out termination.

A requirement therefore exists for an electrostatic developer which enables each line of text to be developed satisfactorily notwithstanding the computer outfeed and irrespective of the stoppage periods.

SUMMARY OF THE INVENTION According to the present invention, there is provided in an electrostatic developer for developing an image on displaceable electrographic paper means for causing developing liquid to contact the paper, and means controlled by the displacement of the paper for varying the quantity of developing liquid contacting the paper such that the paper remains in contact with the liquid for a substantially constant period.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which:

FIG. 1 is a cross-section of an electrostatic developer in accordance with the invention;

FIGS. 2 and 3 are graphs showing the displacement of electrographic paper and of a movable wall of the developer, as a function of time,

FIG. 4 shows schematically means for controlling the displacement of the movable wall;

FIG. 5 shows schematically a modified form of the control means;

FIG. 6 is a cross-section of a second embodiment of the developer;

FIG. 7 is a cross-section of a third embodiment of the developer;

FIG. 8 is a cross-section of a fourth embodiment of the developer;

FIG. 9 is a cross-section of a fifth embodiment of the developer;

FIG. 10 is a section similar to FIG. 9 but showing the developer in a closed, inoperative, condition;

FIG. 11 is a section similar to FIG. 10 and showing the developer moved away from the paper;

FIG. 12 shows schematically feed means for developing liquid; and

FIG. 13 is a cross-section of a sixth embodiment of the developer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 1 electrographic paper 1 having its dielectric layer uppermost is displaced from left to right in the Figure between a printing electrode 2 and a counter-electrode 3 of an electrostatic printing machine. The electrode 2 deposits dielectric charges on the paper 1 in the form of letters, arranged in lines extending at right angles to the plane of the figure.

From the electrode 2, the paper is fed to an electrostatic developer which comprises an enclosure 4 which is open at its lower side and forms a seal with the sheet 1, the latter being supported from beneath by a wall 5 situated at the level of the counter-electrode 3. A wall 6 is displaceable within the enclosure 4 which contains developing liquid 7; the lower surface of the wall 6 forms a scraper arranged to dry the paper at the end of a developing operation. The wall 6 is rigid, for example with a rod 8 having a rack portion meshing with a pinion 9 keyed to a shaft 10 such that rotation of the shaft 10 causes displacement of the wall 6. The enclosure 4 has two lateral openings one of which is shown at 11; the opening 11 is connected by a pipe 12 to a pump 13. The intake of the pump 13 is connected by a pipe 14 to a vessel 15 containing the developing liquid; the other lateral opening of the enclosure 4 is connected to the vessel 15 by a return pipe (not shown). The pump thus renders it possible to establish continuous circulation of the developing liquid 7 through the enclosure 4.

If it is assumed that the printing machine can print ten lines of text per second and that the optimum developing period in 0.5 seconds, the optimum distance between the wall 6 and the opposing surface 4a of the enclosure 4 should correspond to five lines of text. The displacement of the paper 1 is usually, however, intermittent and proceeds line by line, with stopping periods of varying duration. The optimum developing period of 0.5 seconds can be provided for each line, by varying the position of the wall 6 every one-tenth seconds; if the stopping period becomes protracted, the wall 6 is moved towards the left if necessary as far to the opposing surface 4a of the enclosure 4, in which case all of the liquid is displaced from the enclosure 4 and the developing action ceases.

FIGS. 2 and 3 demonstrate the law linking the displacement of the sheet of paper 1 with the displacement of the wall 6. In these figures, the abscissa x of the paper and the distance y between the wall 6 and the surface 4a of the enclosure 4 have been plotted as a function of time. Each time the paper 1 moves onwards by one line (trace 16), the wall 6 advances by one line in the same direction, remains in this position for 0.5

seconds, and then moves back by one line (trace 17). In FIG. 2, the paper I had incurred a stop exceeding 0.5 seconds and the wall 6 was initially in contact with the surface 4a; it has been displaced by one line at the same time as the paper and, after 0.5 seconds, it has been returned into contact with the surface 40.

If the advance of the paper is governed by a more complex irregular rule, it is sufficient to establish the algebraic sum of the displacements of the wall 6 corresponding to each elemental advance of the paper 1, and to plot the graph of the advance of the wall 6, as demonstrated by the traces l6 and 17 of FIG. 3.

In general terms, if x =f(!) represents the displace ment of the paper, that of the wall 6 is represented by:

6 being the development period selected.

If the displacement of the paper is incremental, as in the foregoing example, it is possible to determine the displacement of the wall 6 by means of a shift register 18 and a summator 19 (FIG. 4). The register I8 has n settings, n being the number of lines printed during the optimum developing period and being equal to five in the foregoing example. The register I8 retards and inverts a signal .r representing the abscissa of the paper, and the summator 19 receives the signal x and the signal provided by the shift register 18. A signal y representing the displacement of the wall 6 is obtained at the output of the summator 19. The summator I9 is connected to a shunt circuit 20 which converts the rising portion of the signal y into a pulse which is fed to a reversible motor 21. The motor 21 drives the shaft and thus controls the displacement of the wall 6. The signal is also applied to a second shunt circuit 22 which feeds pulses to a motor 23 controlling the advance of the paper 1.

FIG. 5 shows means for the advancing of the paper 1 and for the displacing of the wall 6, with the aforesaid summation being performed mechanically. In FIG. 5, the output of the shift register is connected via the shunt circuit to an electromagnet 21. The shunt cir cuit 22 is connected to an electroniagnet 24 having a core in the form of a plunger. An clectromagnet 23 actuates a pawl 25 co-operating with a ratchel wheel 26 having a spindle 27 connected to the shaft of a roller driving the paper 1. The electromagnets 21 and 24 actuate respective pawls 28 and 29 which act in opposite directions on a ratchet wheel 30 having a spindle 31 connected to the shaft II) of the pinion 9.

In the embodiment of FIG. 6, the printing electrode 2 is embedded within the thickness of one side wall 32 of the enclosure. a displaceable wall 33 of this enclosure being connected to the stationary part thereof by bellows 34, the wall 33 also being connected to an actuating member 35. The counter-electrode 3 is embedded in the wall 5 supporting the paper II. The thickness of the part of the wall 32 separating the electrodes from the liquid and situated adjacent to the paper 1 is less than the width of the blank spacing separating two lines of text so that the text printed becomes visible as soon as it has been developed, even if the printing machine stops. The actuating member 35 can be displaced in the same manner as the rod 8, for example. The wall 33 and the paper 1 are displaceable vertically.

In a modified form of this embodiment, the member 35 can be omitted and the level of developing liquid 7 is caused to vary by means of a valve interposed in the feed pipe 12 and controlled in a similar manner to the member 35.

In the embodiment of FIG. 7, the paper 1 enters the enclosure 36 through the bottom thereof while the paper is displaced vertically, and a side 37 of the enclosure parallel to the paper is transparent, which renders it possible to see the text as soon as it has been developed. As in the modified form of the embodiment 01 FIG. 6, the level of the developing liquid is varied, by means of a valve interposed in the feed pipe, in such a manner that this level complies with the law stated previously.

In the embodiment of FIG. 8, the paper 1 is displaced in front of a series of nozzles 38 which direct the jets of developing liquid onto the underside of the paper I. A shutter 39 is displaceable in front of the nozzles and renders it possible to intercept all or some of the jets: the shutter 39 is rigid with an actuating member 40 which is moved in a similar manner to the rod 8, for example.

In the embodiment of FIGS. 9 to 11 two printing electrodes 2a and 2b are embedded in the thickness ol one wall 32 of the enclosure, and two counter electrodes 3a and 3b are embedded in the insulated wall 5 supporting the paper 1. The paper 1 is displaced upwardly in front of nozzles 38 which discharge jets ol developing liquid. Each nozzle 38 is directed towards one line of text on the paper. A partition or wall 39 displaceable in front of the nozzles 38 renders it possible to intercept all or some of the jets.

Thejets 38 are connected to the source of developing liquid by a passage 41 formed by a wall 42; the wall 42 is joined to the wall 32 and to lateral walls (not shown) whereby a chamber 43 delimited by the displaceable wall 39 and the wall 5 is approximately sealed. The chamber 43 is in communication with a drain passage 44, the cross-sectional area of which is sufficiently large so that the chamber 43 is never completely filled with liquid. Leakages through the rear joint and through the joints of the lateral walls, are thereby prevented. The upper surface 32a of the wall 32 is inclined towards the drain passage 44 in such manner as to promote the outflow of the developing liquid into the passage 44.

The wall 39 is displaceable as a function of the displacement of the paper in accordance with the rule stated previously, that is:

f(t) representing the displacement of the paper and 6 being the developing period selected, and being of the order of 0.1 to 0.5 seconds. The displacement of the wall 39 can be produced by the arrangement shown in FIG. 5, wherein the spindle 31 carries a pulley 45 around which are wound several turns of a cable 46. The inner turn is secured to the pulley 45 by means ol a'peg 47. The ends of the cable are secured to the wall 39 by two pegs 48 and 49, a tension spring 50 being in terposed in the cable. Pivotal movement of the spindle 31 is thus converted into rectilinear displacement ol the wall 39.

In operation, the jets of liquid issuing from the nozzles 38 strike the paper 1 over a variable width, in such manner that the paper remains in contact with the liq uid for a practically constant period. The coloured particles contained in the liquid remain affixed to the paper at the points at which charges had been deposited by the electrodes 2a and 2b, thus visibly forming the electrostatic image. In FIG. 9, the wall 39 is in its highest position and, for example, uncovers three lines of text which are contacted by one or more rows of jets; in FIG. the wall 39 is in its lowest position and no liquid reaches the paper. The surplus liquid returned to the storage vessel through the drain passage 44.

The part of the wall 39 in contact with the paper 1, carries a smooth scraper 51 which ensures that the paper is dried. The upper surface 39a of the wall 39 slopes downwardly towards the paper, which renders it possible to read the last line of text as soon as the developer is closed; the lower surface 39b of the wall 39 is parallel to the upper surface 32a of the wall 32, in such manner as to ensure an approximately sealed closure when the wall 39 is in its lowest position (FIG. 10).

The inner surface 39c of the wall 39 which partially delimits the chamber 43, is concave so as to form a deflector which directs the jets of liquid towardsthe passage 44 in step with the downward displacement of the wall 39. Shortly before the complete closure of the developer, the jets of developing liquid are totally deflected downwardly towards the passage 44 whereby no liquid contacts the paper; the developing liquid contin ues t0 flow into the chamber 43 at the same flow rate however, and is discharged through the passage 44. This downward deflection of the jets of liquid can alternatively be effected by other suitable walls or projections.

The turbulence thus created leads to satisfactory electrification of the particles by triboelectricity and to homogenous dispersion of the particles in the liquid.

In order to prevent undesirable spots from being formed on the paper along the edges of the chamber 43 in contact with the paper, it is advantageous for the surfaces in contact with the paper to consist of metal whereby the surfaces can be raised to a potential to create a slight degree of repulsion of the particles close to these edges. This is the case for the two lateral walls as well as for the two surfaces 32a and 39b of the walls 32 and 39, of which the metal surfaces are provided by metal inserts 52 and 53.

A groove 54 having end portions open to the surrounding atmosphere is formed in the surface of the wall 32 facing the paper 1 and is located between the electrodes and 2b and the surface 32a of the wall. The groove 54 prevents wetting of the electrodes which would inhibit any recording.

It is advantageous to create a slight negative of the order of 5 to 15 cms of water within the drain passage 44. Leakage of liquid between the developer and the paper is thus prevented, since the pressure within the developer would cause an inflow of air rather than an outflow of liquid. Further, when the developer is closed, it can be displaced from the paper without risk (FIG. 11) which may be necessary if it is necessary to cause the paper to perform a very quick jump between two lines of print, as occurs in printers employing mechanical impression; this displacement also enables the last line developed to be easily read.

FIG. 12 shows suitable means for creating the negative pressure and for feeding the developing ink. The developing liquid 7 is enclosed in a tank 55 sealed by a filler plug 56. The inside of a pump 57 of the submerged type is connected to the base ofthe tank 55 whereas its discharge is connectedv to the passage 41. The drain passage 44 leads to the upper part of the tank 55. An air pump 58, having an intake connected to the upper part of the tank 55, generates a negative pressure in the tank which is transmitted into the chamber 43 of the developer. In order to reduce to a minimum pollution of the surrounding atmosphere by the vapours of the developing liquid, the negative pressure should be as low as possible; a pressure of 6 cms water will provide satisfactory results. I

In the embodiment of FIG. 13, the wall 39 is replaced by a wall which is arranged to pivot on the wall 42. The wall 60 is pivotally and sealingly mounted on the wall 42 by means of an elastic packing 61 which is held lightly compressed by the wall 60 under the action of a roller 62. The roller 62 may be stationary or can be rotatable around its axis. This arrangement enables the small longidudinal displacement of the wall 60, due to the linear trajectory of its scraping edge against the paper 1, to be taken up. A spring 63 having one end is fastened to the wall 42 and the other end in contact with the edge of the wall 60, ensures constant contact between the wall 60 and the paper, irrespective of its position, which ensures effective drying.

A spring 64 keeps the wall 60 in contact with a cam 65 rigid with the spindle 31; as shown, the cam 65 is located inside the developing chamber 43, but the cam could be situated outside the chamber.

The nozzles 38 consist of one or more rows of orifices situated in a distributor 41. The wall 60 is rigid with a deflector 66 which is engaged by the jets when the wall 60 is in the closed position and which deflects the liquid towards the drain passage 44.

The walls 60 and 67 which delimit the chamber 43 consist of metal and are raised to a potential sufficient to eliminate any background and interference spots.

A conductive wall 68 may be inset into the insulating wall 5 to establish the correct potential on the back of the paper during the developing action. This is particularly useful if the counter-electrodes 3 are segmented and brought to widely varying potentials. I

It will be apparent from the preceding description that the developers of FIGS. 9 to 13 form a single unit with the printing electrodes to produce a selfdeveloping recording head. When the head is closed, it can be moved away from the paper to facilitate perusal or rapid displacement of the paper; it does not allow any liquid to escape to the outside, the liquid nevertheless continuing to circulate normally inside the same.

What is claimed is:

1. ln an electrostatic developer for developing an image on displaceable electrographic paper means for causing developing liquid to contact the means controlled by the displacement of the paper for varying the length of paper contacted by the developing liquid, and

means for drying the paper when it loses its contact with the liquid, such that the paper remains in contact with the liquid for a substantially constant period.

2. A developer according to claim 1,'wherein the means for causing developing liquid to contact the paper comprises means defining a chamber for the liquid, said chamber defining means including a wall extending transversely with respect to the paper and movable in the direction of displacement of the paper, and the said means for varying the quantity of liquid contacting the paper comprises means for displacing the wall as a function of the displacement of the paper, the said chamber having an opening closable by the paper.

3. A developer according to claim 2, wherein the wall has a surface which forms a scraper for drying the paper at the end of the developing process.

4. A developer according to claim 1, wherein the means for causing developing liquid to contact the paper comprises means defining a chamber for the liquid, the paper being arranged to pass through the liquid in the chamber, and the means for varying the quantity of liquid contacting the paper comprises means for varying the level of the liquid in the chamber as a function of the displacement of the paper.

5. A developer according to claim 1, wherein the means for causing developing liquid to contact the paper comprises a series of nozzles arranged to direct the liquid towards the paper, and the means for varying the quantity of liquid contacting the paper comprises an interceptor element movable in front of these nozzles controlled by the displacement of the paper.

6. A developer according to claim 1 wherein the means for varying the quantity of liquid contacting the paper comprises a member displaceable controlled by displacement of the paper, and

means for imparting to the displaceable member a displacement represented by the equation y =f(t) f(r wherein y represents the abscissa of the displacement member and 0 the optimum developing period, the abscissa x of the paper being given by the equation x =f(t).

7. A developer according to claim 6 wherein the means for displacing the said member comprises a shift register having n settings, n being the maximum number of lines printed during the optimum developing period, said register being arranged to retard and invert a signal representing the abscissa of the paper, and

a summator which receives the said signal and an output signal from the shift register.

8. A developer according to claim 7, wherein the means for displacing the said member further comprises a shunt circuit, and

a reversible stepping motor, the summator being connected through the shunt circuit to the reversible motor, said motor controlling the displacement of the displaceable member.

9. A developer according to claim 7 wherein the sum mator comprises a ratchet wheel, and the said means for displacing the member further comprises two pawls acting on the ratchet wheel in opposite directions a respective electromagnet controlling each pawl,

a first shunt circuit, one of the electromagnets being controlled by the shift register through the first shunt circuit, and

a second shunt circuit, the other electromagnet being controlled by the second shunt circuit which receives the signal representing the abscissa of the paper.

10. A developer according to claim 4, wherein the said means for varying the level of the liquid imparts to the liquid a level y represented by the equation:

y=f( )-f( wherein 6 is the optimum developing period, the abscissa x of the paper being given by the equation x f0).

11. A developer according to claim 10 wherein 10 means for varying the level of the liquid comprises a shift register having n settings, n being the maximum number of lines printed during the optimum developing period, said register being arranged to retard and invert a signal representing the abscissa of the paper, and

a summator which receives the said signal and an output signal from the shift register.

12. A developer according to claim 11, wherein the means for varying the level of the liquid further comprises a shunt circuit,

a reversible stepping motor, the summator being connected through the shunt circuit to the reversible motor,

a feed circuit for the liquid and a valve incorporated in the feed circuit, the said motor controlling the valve.

13. A developer according to claim 11, wherein the summator comprises a ratchet wheel, and the said means for varying the level of the liquid further comprises two pawls acting on the ratchet wheel in opposite directions a respective electromagnet controlling each pawl,

a first shunt circuit, one of the electromagnets being controlled by the shift register through the first shunt circuit, and

a second shunt circuit, the other electromagnet being controlled by the second shunt circuit which receives the signal representing the abscissa of the paper. 14. A developer according to claim 1, further comprising means for circulating continuously the developing liquid. 15. A developer according to claim 5 further comprising an insulated supporting wall arranged to support the paper, said insulated wall being inclined to the horizontal,

means defining a chamber into which the nozzles discharge, said chamber being defined by a first insulated wall a second wall and,

a third wall, the third wall being movable with respect to the first and second walls and forming the interceptor element,

means defining a drain passage from the chamber for the developing liquid, and

co-operating electrodes inset in the said supporting wall and the said first wall of the chamber.

16. A developer according to claim 15, wherein the second wall carries the nozzles.

17. A developer according to claim 15, wherein the drain passage is defined by the first and second walls.

18. A developer according to claim 15, wherein the third wall is displaceable rectilinearly.

19. A developer according to claim 15, wherein the third wall is pivotally mounted.

20. A developer according to claim 15, wherein the supporting wall and the drain passage are substantially vertical.

21. A developer according to claim 15, wherein the first wall has a surface situated downstream of the electrodes, said surface being inclined towards the drain passage.

22. A developer according to claim 15, wherein the first and third walls have parallel edges, the third wall being movable to engage the said edges whereby to close the chamber.

23. A developer according to claim 15, further comprising a source of developing liquid, and

a passage formed in the second wall and communicating the nozzles with the said source of liquid.

24. A developer according to claim 15 further comprising a scraper arranged to contact the paper, the scraper being carried by the third wall.

25. A developer according to claim 15, wherein the third wall has a concave inner surface arranged to deflect towards the drain passage jets of liquid discharged by the nozzles.

26. A developer according to claim 15, wherein the chamber has edges arranged to contact the paper, the said edges being insulated.

27. A developer according to claim 15 wherein the chamber has edges arranged to contact the paper, the said edges being metallic and being raised to a predetermined potential.

28. A developer according to claim 15 wherein a surface of the first wall housing the electrodes contains a groove arranged to prevent wetting of the electrodes.

29. A developer according to claim 15, further comprising means for generating a negative pressure in the drain passage.

30. A developer according to claim 15, wherein the drain passage has a cross-section which is sufficient to permit draining of all of the developing liquid discharged by the nozzles. 

1. In an electrostatic developer for developing an image on displaceable electrographic paper means for causing developing liquid to contact the paper, means controlled by the displacement of the paper for varying the length of paper contacted by the developing liquid, and means for drying the paper when it loses its contact with the liquid, such that the paper remains in contact with the liquid for a substantially constant period.
 2. A developer according to claim 1, wherein the means for causing developing liquid to contact the paper comprises means defining a chamber for the liquid, said chamber defining means including a wall extending transversely with respect to the paper and movable in the direction of displacement of the paper, and the said means for varying the quantity of liquid contacting the paper comprises means for displacing the wall as a function of the displacement of the paper, the said chamber having an opening closable by the paper.
 3. A developer according to claim 2, wherein the wall has a surface which forms a scraper for drying the paper at the end of the developing process.
 4. A developer according to claim 1, wherein the means for causing developing liquid to contact the paper comprises means defining a chamber for the liquid, the paper being arranged to pass through the liquid in the chamber, and the means for varying the quantity of liquid contacting the paper comprises means for varying the level of the liquid in the chamber as a function of the displacement of the paper.
 5. A developer according to claim 1, wherein the means for causing developing liquid to contact the paper comprises a series of nozzles arranged to direct the liquid towards the paper, and the means for varying the quantity of liquid contacting the paper comprises an interceptor element movable in front of these nozzles controlled by the displacement of the paper.
 6. A developer according to claim 1 wherein the means for varying the quantity of liquid contacting the paper comprises a member displaceable controlled by displacement of the paper, and means for imparting to the displaceable member a displacement represented by the equation y f(t) - f(t - theta ), wherein y represents the abscissa of the displacement member and theta the optimum developing period, the abscissa x of the paper being given by the equation x f(t).
 7. A developer according to claim 6 wherein the means for displacing the said member comprises a shift register having n settings, n being the maximum number of lines printed during the optimum developing period, said register being arranged to retard and invert a signal representing the abscissa of the paper, and a summator which receives the said signal and an output signal from the shift register.
 8. A developer according to claim 7, wherein the means for displacing the said member further comprises a shunt circuit, and a reversible stepping motor, the summator being connected through the shunt circuit to the reversible motor, said motor controlling the displacement of the displaceable member.
 9. A developer according to claim 7 wherein the summator comprises a ratchet wheel, and the said means for displacing the member further comprises two pawls acting on the ratchet wheel in opposite directions a respective electromagnet controlling each pawl, a first shunt circuit, one of the electromagnets being controlled by the shift register through the first shunt circuit, and a second shunt circuit, the other electromagnet being controlled by the second shunt circuit which receives the signal representing the abscissa of the paper.
 10. A developer according to claim 4, wherein the said means for varying the level of the liquid imparts to the liquid a level y represented by the equation: y f(t) - f(t - theta ) wherein theta is the optimum developing period, the abscissa x of the paper being given by the equation x f(t).
 11. A developer according to claim 10 wherein means for varying the level of the liquid comprises a shift register having n settings, n being the maximum number of lines printed during the optimum developing period, said register being arranged to retard and invert a signal representing the abscissa of the paper, and a summator which receives the said signal and an output signal from the shift register.
 12. A developer according to claim 11, wherein the means for varying the level of the liquid further comprises a shunt circuit, a reversible stepping motor, the summator being connected through the shunt circuit to the reversible motor, a feed circuit for the liquid and a valve incorporated in the feed circuit, the said motor controlling the valve.
 13. A developer according to claim 11, wherein the summator comprises a ratchet wheel, and the said means for varying the level of the liquid further comprises two pawls acting on the ratchet wheel in opposite directions a respective electromagnet controlling each pawl, a first shunt circuit, one of the electromagnets being controlled by the shift register through the first shunt circuit, and a second shunt circuit, the other electromagnet being controlled by the second shunt circuit which receives the signal representing the abscissa of the paper.
 14. A developer according to claim 1, further comprising means for circulating continuously the developing liquid.
 15. A developer according to claim 5 further comprising an insulated supporting wall arranged to support the paper, said insulated wall being inclined to the horizontal, means defining a chamber into which the nozzles discharge, said chamber being defined by a first insulated wall a second wall and, a third wall, the third wall being movable with respect to the first and second walls and forming the interceptor element, means defining a drain passage from the chamber for the developing liquid, and co-operating electrodes inset in the said supporting wall and the said first wall of the chamber.
 16. A developer according to claim 15, wherein the second wall carries the nozzles.
 17. A developer according to claim 15, wherein the drain passage is defined by the first and second walls.
 18. A developer according to claim 15, wherein the third wall is displaceable rectilinearly.
 19. A developer according to claim 15, wherein the third wall is pivotally mounted.
 20. A developer according to claim 15, wherein the supporting wall and the drain passage are substantially vertical.
 21. A developer according to claim 15, wherein the first wall has a surface situated downstream of the electrodes, said surface being inclined towards the drain passage.
 22. A developer according to claim 15, wherein the first and third walls have parallel edges, the third wall being movable to engage the said edges whereby to close the chamber.
 23. A developer according to claim 15, further comprising a source of developing liquid, and a passage formed in the second wall and communicating the nozZles with the said source of liquid.
 24. A developer according to claim 15 further comprising a scraper arranged to contact the paper, the scraper being carried by the third wall.
 25. A developer according to claim 15, wherein the third wall has a concave inner surface arranged to deflect towards the drain passage jets of liquid discharged by the nozzles.
 26. A developer according to claim 15, wherein the chamber has edges arranged to contact the paper, the said edges being insulated.
 27. A developer according to claim 15 wherein the chamber has edges arranged to contact the paper, the said edges being metallic and being raised to a predetermined potential.
 28. A developer according to claim 15 wherein a surface of the first wall housing the electrodes contains a groove arranged to prevent wetting of the electrodes.
 29. A developer according to claim 15, further comprising means for generating a negative pressure in the drain passage.
 30. A developer according to claim 15, wherein the drain passage has a cross-section which is sufficient to permit draining of all of the developing liquid discharged by the nozzles. 